1858.] on the Rhizopod Type of Animal Life, 501 



same specific type. It is very easy to select from any extensive 

 collection of foraminifera, recent or fossil, sets of forms having 

 certain characters in common, but yet so dissimilar in other respects 

 that few naturalists would have any doubt as to their specific or 

 even generic distinctness ; yet when the collection is thoroughly 

 examined, such a series of intermediate forms is found to exist, as 

 connects all these by gradations so insensible as to prevent the pos- 

 sibility of any line of demarcation being satisfactorily drawn between 

 them. A remarkable example of this kind is presented by the 

 generic types designated as Dendritina and Peneroplis ; the former 

 being a minute shell, resembling that of the nautilus in its general 

 proportions, and having a single large dendritic aperture in its suc- 

 cessive partitions ; whilst the latter is flattened, and instead of one 

 large aperture, has a series of small foramina arranged in a single 

 line. Now between these every gradation can be found, both in 

 the form of the shell and in the mode of communication through the 

 septa ; the flattened shell of Peneroplis presenting various degrees 

 of turgidity until it attains the proportions of Dendritina ; and the 

 linear arrangement of the isolated apertures, in like manner, giving 

 place to one in which they are approximated more closely together 

 into a sort of bundle, still, however, retaining their distinctness ; 

 whilst in other individuals, the distinct apertures coalesce into one 

 large jagged orifice, the borders of which become more and more 

 deeply cut, until they present the ramifying extensions characteristic 

 of dendritina. Now if, in such a series, we once begin to make a 

 distinct species for every well marked dissimilarity, either in the 

 form of the shell, or in that of the aperture, we must multiply our 

 species almost indefinitely, contrary to all probability ; and there 

 is no medium between doing this, and uniting the whole series of 

 forms included in these two reputed genera under one specific type. 

 This is the more remarkable, because in one locality we may find 

 only the Dendritina-form, in another only the Peneroplis-form, 

 whilst the transitional or intermediate forms come from a third. 



Another remarkable example of this wide range of specific 

 characters is presented in the Orhitolite, a composite organism, 

 which, originating in a spheroidal nucleus of sarcode, increa^ by 

 the formation of new segments in concentric rings around this, so 

 that, each segment becoming invested with a shelly envelope, a very 

 beautiful disk is formed, which is enlarged by successive additions 

 to its margin. The segments communicate with each other by 

 annular canals ; and there are also passages connecting each annu- 

 lus with those within and without ; whilst from the outermost an- 

 nul us there are passages opening at the margin of the shelly disk, 

 through which alone the pseudopodia issue that obtain the food for 

 the whole organism. Now there are two very distinct types of 

 growth presented by these Orbitolites ; one, namely, in which the 

 disk is very thin, and the segments form (as it were) but a single 

 floor ; and the other in which the disk becomes comparatively thick 



